From the aforedescribed copending applications and other publications cited therein, it will be apparent that electronic, contactless switching devices have been provided heretofore and, generally, are known for a variety of applications.
It is also known to provide switching devices in a housing, generally a two-part housing, in which a cover is removably mounted upon a base which can be formed with a connection device electrically joined, in turn, to a cable which may pass through a pipe, duct or conduit threaded into the housing. The switching circuitry, in turn, can be provided in the receptacle formed by the base and can be electrically tied to the connecting or terminal device. The terminal device can, for example, be a terminal block and the switching circuitry can include at least one sensing element, for example, a sensing coil, a sensing electrode or a sensing opto-electronic system which can include a light generator and a light receiver.
Generally speaking, all of the circuit elements are provided in the receptacle formed by the bottom portion of the housing and are rendered accessible by removal of the cover.
Switching devices of the aforedescribed type are available in a variety of designs and for various purposes. For example, they may be proximity switches of the type described in the aforementioned copending application or, if required, contact switches which respond to the slightest of contact by the hand of the user or by some other element. Devices of the latter type are well known in elevator applications and the like.
The housing and fastening systems for such switches may correspond to various governmental standards, for example, the German industrial standard (DIN) 43 694.
In such a system, the bottom portion of the housing is generally block-shaped, i.e. has the configuration of a rectangular parallelepipedal box, and is provided with two receptacles. A first of these receptacles receives the terminal device, i.e. the portion of the electrical circuit provided with screw or other terminals, while the second receptacle encloses or receives the electronic switching circuit, or, more generally, the switching device. The upper portion of the housing forms a cover which seals the two receptacles of the bottom portion of the housing.
In this connection reference may be had to the German utility models (Gebrauchsmuster) 70 29 767 and 74 40 132.
Common to both these systems is an organization of the device whereby a replacement of the electrical circuit unit is possible although it is relatively expensive and complicated.